Duchenne/Becker muscular dystrophy (DMD/BMD) is a genetic disorder that develops in boys. It is caused by a mutation in the gene for dystrophin, a protein that is important for maintaining normal muscle structure and function. Loss of dystrophin causes muscle fragility that leads to weakness and loss of walking ability during childhood and teenage years. A specific type of mutation, called a nonsense (premature stop codon) mutation is the cause of DMD/BMD in approximately 10-15% of boys with the disease. Ataluren (PTC124) is an orally delivered, investigational drug that has the potential to overcome the effects of the nonsense mutation. This study is a Phase 2b extension trial that will evaluate the long-term safety of ataluren (PTC124) in boys with nonsense mutation DMD/BMD, as determined by adverse events and laboratory abnormalities. The study will also assess changes in walking, muscle function, and other important clinical and laboratory measures.